RESEARCH METHODS-QUALITATIVE STUDY DESIGNS-BASING ON NATURE OF THE INVESTIGATION-THE DOUBLE-CONTROL DESIGN

Although the control design helps you to quantify the impact that can be attributed to extraneous variables, it does not separate out other effects that may be due to the research instrument (such as the reactive effect) or respondents (such as the maturation or regression effects, or placebo effect). When you need to identify and separate out these effects, a double-control design is required.  In double-control studies, you have two control groups instead of one. To quantify, say, the reactive effect of an instrument, you exclude one of the control groups from the ‘before’ observation.